1. Field of the Invention
The present invention relates generally to insulation apparatus for electrical components, and more particularly relates to insulative apparatus for supporting an electrical component in a manner maintaining a predetermined minimum air space clearance length between the supported component and a closely adjacent metal structure such as a heat sink plate.
2. Description of Related Art
Conformance to Underwriter's Laboratories (UL) standards, and various international electrical codes and standards, typically requires that a minimum effective air space clearance length be maintained between certain relatively high voltage electrical components and adjacent electrically conductive material such as metal heat sink structures upon which the components are mounted. As an example of these standards, it is typically required that all surface portions of electrical components handling at least 300 volts but less than 400 volts be spaced apart from adjacent electrically conductive structures by an effective minimum air path length (or an equivalent, lesser path length through a solid insulative material) of 8 mm. Higher voltage components are required to have correspondingly greater minimum effective air path clearance lengths to the adjacent electrically conductive structures. The purpose of these minimum spacing requirements is to guard against potentially dangerous short circuiting between the components and the adjacent electrically conductive structures.
On its face, at least, this seems a relatively simple safety standard to comply with. However, various difficulties have been encountered in uniformly adhering to this spacing standard due, primarily, to the conventional structures and methods commonly utilized to support the electrical components adjacent electrically conductive structures from which they must be at least minimally isolated.
One commonly encountered situation in which uniform adherence to this spacing standard is particularly difficult is where the component is, for heat dissipation purposes, secured to a metal heat sink member. A typical method of doing this is to encapsulate the high voltage component in an insulating shroud, such as a silicon rubber encapsulating tube, and then compressively clamp the shrouded component against the metal heat sink. The purpose of maintaining a compressive force on the shrouded component is twofold. Up to a certain point, such compressive force desirably increases the dielectric strength (i.e., the electrical resistance) of the insulating material while at the same time increasing its thermal conductivity.
The use of silicon rubber encapsulating material, while a relatively simple and inexpensive approach to the electrical isolation task at hand, is not entirely satisfactory for two primary reasons. First, as is well known, from an electrical insulating standpoint silicon rubber is typically considered to be a low voltage insulating material, and is not particularly well suited to high voltage insulating applications. Second, silicon rubber is not a particularly hardy material and tends to be rather easily torn during installation and subsequent handling. Once this insulation material is torn, of course, the possibility arises that a dangerously short air spark path will be formed between the shrouded component and the adjacent metal heat sink.
Another conventional method utilized to mount high voltage electrical components on a metal heat sink structure is to provide a spaced series of essentially planar isolating pads, each formed from a suitable rigid insulating material such as a ceramic material. The spaced apart pads are positioned flat against a side surface of the heat sink, the electrical components are positioned against the outer sides of their associated pads, and the in-place components are pressed against their pads (utilizing a suitable clamping mechanism), thereby exerting the desired compressive forces on both the components and their underlying pads.
Like its silicon rubber shroud counterpart, this isolation technique has not proven to be wholly satisfactory in practice for two primary reasons. First, to properly position the components and their associated insulation/isolation pads on the heat sink structure, and relative to one another, it has typically been necessary to employ a significant amount of assembly fixturing. This necessity, of course, tends to undesirably increase the overall fabrication cost of the finished product.
Second, a heretofore unavoidable characteristic of this component support and isolation technique is that the electrical components tend to shift or "creep" on their underlying pads toward a side edge portion thereof. The closer the shifting component gets to such pad side edge, the shorter the minimum effective air gap length between the component and the underlying metal heat sink structure becomes, the minimum length of such air gap being geometrically dependent on the proximity between the supported component and such pad side edge. In many cases, even a relatively small, undiscernible shift of the component can create a potentially dangerous reduction in the minimum effective isolation distance between the component and the metal heat sink structure.
It can be readily seen from the foregoing that it would be highly desirable from a safety standpoint to provide improved apparatus and methods for supporting high voltage electrical components in a thermally conductive relationship with, yet in uniformly maintained spatial isolation from an electrically conductive structure such as a metal heat sink member. It is accordingly an object of the present invention to provide such apparatus and methods.